Method for separating salified phenolic compounds

ABSTRACT

A method for separating a salified phenolic compound from a reaction medium including same is described. Also described, is a method for separating salified phenolic compounds from an aqueous reaction medium resulting from the reaction of a phenolic compound and glyoxylic acid in the presence of a base, which leads to a reaction medium including at least the excess of the starting salified phenolic compound and the various salified mandelic compounds resulting from the reaction, wherein the reaction medium including the starting salified phenolic compound is contacted with an adsorbent substrate. This leads to the selective adsorption of the phenolic compound onto said substrate and to the recovery of an aqueous flow containing the salified mandelic compounds from the reaction, and in that the phenolic compound attached onto the adsorbent is desorbed by means of a regenerating treatment of the adsorbent.

The present invention provides a method of separating phenolic compoundsin salified form from a reaction mixture comprising them.

More specifically, the invention relates to the separation of guaiacolor of guaethol in salified form from the synthesis mixtures comprisingthem.

The invention is directed more particularly to the recovery of guaiacolin the form of sodium salt, present in excess amount, during thesynthesis of vanillin or 4-hydroxy-3-methoxybenzaldehyde.

Hydroxyaromatic and alkoxyaromatic aldehydes are very importantproducts, which are used as flavors and fragrances and as intermediatesin numerous fields, such as, for example, agrochemicals, pharmacy,cosmetology, and other industries.

The ortho- and para-hydroxybenzaldehydes,4-hydroxy-3-methoxybenzaldehyde and 3-ethoxy-4-hydroxybenzaldehyde,named “vanillin” and “ethylvanillin” respectively, are among the mostimportant products.

Various processes have been proposed for the synthesis of aromaticaldehydes.

The most important processes are based on the functionalization of aphenolic starting compound, phenol, catechol derivative, guaiacol (or2-methoxy-phenol), guaethol (or 2-ethoxyphenol).

In this type of process, the phenolic compound is generally involved ina salified form, for example, in the form of a sodium salt.

Thus, for example, numerous processes for preparing vanillin involve aguaiacol salt as substrate, to which is then added a formyl group, inthe position para to the hydroxyl group, by various methods.

One conventional route to vanillin involves a condensation reaction ofglyoxylic acid with guaiacol, in basic medium, to give4-hydroxy-3-methoxymandelic acid. This product is then oxidized toproduce vanillin.

The reaction is commonly conducted in the presence of sodium hydroxideand with an excess of guaiacol, with glyoxylic acid being the deficitreactant.

Thus, at the end of the condensation reaction, an aqueous reactionmixture is obtained that comprises the sodium salt of4-hydroxy-3-methoxymandelic acid, the precursor to vanillin, secondaryproducts, such as the sodium salts of 2-hydroxy-3-methoxymandelic acidand 4-hydroxy-5-methoxy-1,3-dimandelic acid, and a greater or lesserexcess of sodium guaiacolate.

In this reaction mixture, therefore, there are a number of types ofsalified phenolic compounds present, namely guaiacol in excess in theform of sodium guaiacolate, and the products of the reaction which arealso salified phenolic compounds, such as the sodium salts of4-hydroxy-3-methoxymandelic acid, 2-hydroxy-3-methoxymandelic acid, and4-hydroxy-5-methoxy-1,3-dimandelic acid.

For economic reasons it is important to recover the unreacted startingsubstrate. However, the operation is not easy, since the guaiacol is inthe form of sodium guaiacolate and is present alongside phenoliccompounds which are also salified and have a closely related structure.

In certain processes described in the prior art, especially in FR 2 132364, the sodium guaiacolate, at the end of the condensation reaction, isconverted to guaiacol by an acid treatment, most often with sulfuricacid.

The unconverted guaiacol is then extracted from the acid solution by anextraction treatment using a hydrocarbon, for example, benzene ortoluene.

The drawback of a method of this kind is that it employs an organicsolvent, thereby giving rise to additional distillation operations inorder to be able to recycle the organic solvent and the substraterecovered. Moreover, in the course of the distillation, there aresecondary reactions which lead to the formation of heavy products.

Furthermore, the neutralization of sodium guaiacolate with sulfuric acidproduces sodium sulfate, leading to the formation of substantial salteffluents.

Moreover, the guaiacol recovered must be salified again in order to beintroduced into the condensation reaction with glyoxylic acid.

Similarly, the reaction mixture, comprising the mandelic compounds witha free hydroxyl group, must be salified again in order to be introducedinto the oxidation reaction that allows vanillin to be obtained.

In order to overcome these drawbacks, the invention provides a methodthat allows the excess of phenolic starting compound in salified form,especially sodium guaiacolate, to be recovered, by a method which doesnot involve this step of neutralizing sodium guaiacolate to guaiacol,with the attendant need for said guaiacol to be extracted using anorganic solvent which must subsequently be separated by distillation.

A method has now been found, and constitutes the subject of the presentinvention, of separating phenolic compounds in salified form from anaqueous reaction mixture resulting from the reaction of a phenoliccompound and glyoxylic acid in the presence of a base, leading to areaction mixture comprising at least the excess of phenolic startingcompound in salified form and the various mandelic compounds in salifiedform, resulting from the reaction, characterized in that said reactionmixture comprising the phenolic starting compound in salified form iscontacted with an adsorbent support, leading to the selective adsorptionof said phenolic compound on said support, and to the recovery of anaqueous stream comprising the mandelic compounds in salified formobtained from the reaction, and in that the phenolic compound attachedto the adsorbent is desorbed by a regenerative treatment of saidadsorbent.

In the description below of the present invention, the term “phenolicstarting compound” means a benzene compound in which at least onehydrogen atom directly bonded to the benzene nucleus is substituted by ahydroxyl group.

In accordance with the method of the invention, it has been found, inthe case of the treatment of an aqueous, reaction mixture comprisingsodium guaiacolate, that the latter can be adsorbed on the adsorbentsubstrate thereby allowing it to be separated and subsequently, afterdesorption, recycled to the synthesis step without the need to pass viaa step of acidification of the sodium guaiacolate in order to convert itto guaiacol, which is recovered and then subjected to a further basictreatment, since the compound employed in the condensation step is aphenolate.

In order to illustrate the method of the invention, the Applicant isciting the case of the separation of sodium guaiacolate from the aqueousmixture from condensation of sodium guaiacolate and glyoxylic acid.

However, the method of the invention is not limited to the separation ofthis substrate, and is also suitable for phenolic starting compounds insalified form corresponding to the following formula:

in which formula:

-   -   R is an alkyl or alkoxy group having from 1 to 4 carbon atoms,        or a halogen atom,    -   x is a number from 0 to 3, and more preferably is 1, and    -   M represents a cation of a metallic element from group (IA) of        the periodic table, namely lithium, sodium, potassium, rubidium,        and cesium, or an ammonium cation.

In the formula (I), M is preferably sodium.

Examples of alkyl groups that may be mentioned include linear orbranched alkyl groups having from 1 to 4 carbon atoms, such as methyl,ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, andtert-butyl. Among these, the methyl and ethyl groups are preferred.

Examples of linear or branched alkoxy groups having from 1 to 4 carbonatoms include methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy,isobutoxy, and sec-butoxy groups. Methoxy and ethoxy groups arepreferred.

R also represents a halogen atom, preferably fluorine, chlorine, andbromine, and more preferably fluorine.

With regard to the nature of R, it should be noted that the list ofsubstituents given is not limitative, and other substituents may beenvisaged insofar as they do not disrupt the separation of the compoundof formula (I).

Illustrative instances of compounds corresponding to the formula (I)include, more particularly, the salts of compounds selected from thegroup consisting of the following: phenol, guaiacol, 3-methoxyphenol,guaethol, 3-ethoxyphenol, 2-isopropoxyphenol, 3-isopropoxyphenol,2-methoxy-5-methylphenol, 2-methoxy-6-methylphenol,2-methoxy-6-tert-butylphenol, 3-chloro-5-methoxyphenol,2,3-dimethoxy-5-methylphenol, 2,3-dimethoxyphenol, 2,6-dimethoxyphenol,3,5-dimethoxyphenol, cresols, tert-butylphenol, 2-methoxyphenol, and4-methoxyphenol.

The preferred compounds of formula (I) are phenol, guaiacol, andguaethol.

According to the invention, the method of the invention is applied tothe aqueous reaction mixture obtained from the reaction of a phenoliccompound in salified form of formula (I) and glyoxylic acid.

The condensation reaction of the phenolic compound of formula (I) andglyoxylic acid may be conducted in the presence of an ammoniumhydroxide, but more preferably in the presence of an alkali metalhydroxide, which may be sodium hydroxide or potassium hydroxide. Foreconomic reasons it is preferred to select sodium hydroxide.

With regard to the glyoxylic acid, an aqueous solution of glyoxylic acidis employed that has a concentration of, for example, between 15% and70% by weight.

The glyoxylic acid is reacted with the phenolic compound of formula (I)in excess. The molar ratio between the phenolic compound of formula (I)and the glyoxylic acid is between 1.1 and 4.0, preferably between 1.5and 3.0.

The alkali metal hydroxide solution employed has a concentration ofgenerally between 10% and 50% by weight.

The amount of alkali metal hydroxide introduced into the reactionmixture takes account of the amount needed in order to salify thehydroxyl function of the phenolic compound of formula (I), and theamount needed to salify the carboxyl function of the glyoxylic acid.

The concentration of the phenolic compound of formula (I) is preferablybetween 0.5 and 1.5 mol/liter.

The temperature of the reaction is selected advantageously between 20°C. and 60° C.

The reaction is conducted at atmospheric pressure but under a controlledatmosphere of inert gases, preferably nitrogen or rare gases, especiallyargon. It is preferred to select nitrogen.

After the phenolic compound of formula (I) has been contacted with theglyoxylic acid and the alkali metal hydroxide, the reaction mixture ismaintained with stirring and at the temperature selected from theaforementioned range, for a variable duration of from 1 to 10 hours.

At the end of the reaction, an aqueous reaction mixture is obtained thatcomprises the excess of phenolic compound in salified form correspondingto the formula (I) and various mandelic compounds in salified form,denoted by the expression “mandelic compounds” and corresponding to thefollowing formulae:

in which formulae M, R, and x are as defined for the formula (I).

The preferred mandelic compounds correspond to the formulae (IIa),(IIb), and (IIc), in which M represents a sodium atom, x is a numberfrom 0 to 3, and preferably is 1, and the groups R, which are identicalor different, represent an alkyl or alkoxy group having from 1 to 4carbon atoms, preferably a methoxy or ethoxy group, or a halogen atom.

The invention is applied more particularly, in the context of thepreparation of vanillin, to an aqueous mixture comprising sodiumguaiacolate and mandelic compounds in salified form:4-hydroxy-3-methoxymandelic acid, 2-hydroxy-3-methoxymandelic acid,4-hydroxy-5-methoxy-1,3-dimandelic acid, in salified form.

The invention is also applied preferably, for the preparation ofethylvanillin, to a reaction mixture which is an aqueous mixturecomprising sodium guaetholate and mandelic compounds in salified form:3-ethoxy-4-hydroxymandelic acid, 3-ethoxy-2-hydroxy-mandelic acid,5-ethoxy-4-hydroxy-1,3-dimandelic acid, in salified form.

The concentration by weight of the phenolic starting compound insalified form, preferably sodium guaiacolate or sodium guaetholate,varies generally between 1% and 20% by weight, preferably between 5% and10% by weight.

The concentration by weight of mandelic compounds (o-, p-, anddi-mandelate) in the reaction mixture is typically between 3% and 30% byweight, preferably between 5% and 20% by weight.

The method of the invention of separating the phenolic compound insalified form is therefore implemented on the aqueous reaction mixtureas defined above.

In the case of the preparation of vanillin and of ethylvanillin, themixture therefore comprises sodium guaiacolate or sodium guaetholate andvarious reaction products, namely monofunctional or difunctional sodiummandelates as specified in accordance with formulae (IIa), (IIb), and(IIc), predominantly the compound of formula (IIb).

In accordance with the method of the invention, the reaction mixture asdescribed above is contacted with an adsorbent support, leading to theselective adsorption on the support of the phenolic compound, and to therecovery of an aqueous stream comprising the various mandelic compounds,and then the phenolic compound is desorbed by regeneration of theadsorbent support by an appropriate means, preferably using a base.

An adsorbent support is a solid support capable of adsorbing on itssurface the molecules of a substrate referred to as the “adsorbate” byVan Der Waals bonding.

Throughout the rest of the text, the adsorbent support will be referredto more simply as “adsorbent”.

The adsorbents are generally supports which have a very high specificsurface area and exhibit an internal porosity.

The specific surface areas indicated refer to a specific surface areadetermined by the method of BRUNEAU-EMMETT-TELLER as described in “TheJournal of American Society 60, 309 (1938)”.

The adsorbent must be selected with an eye to a number of requirements.

The adsorbent must be physically and chemically stable toward theoperating conditions.

The adsorbent must have a high capacity to adsorb the adsorbate.

The efficiency of the adsorption is measured as a percentage of theadsorbate relative to the mass of the adsorbent. It is situatedgenerally between 10% and 50% by weight.

The adsorbent must have a high selectivity of adsorption for thephenolic compound relative to the mandelic compounds.

The adsorbent must be easy to regenerate, which means that the adsorbatemust be easily desorbed from the adsorbent support.

Adsorbents which can be used in the method of the invention include, inparticular, activated carbons, adsorbent polymers, zeolites, andmolecular sieves.

The adsorbent used according to the present invention may be anactivated carbon.

Numerous grades of active carbons exist, and the carbons which may beused include “physical” carbons, which result from a step ofhigh-temperature calcining of a carbon-containing raw material,generally followed by a step of thermal activation for enhancing theadsorbency of said material.

The active carbon may be based on coal, peat, lignite or petroleumdistillation residues, or on the basis of any carbon-rich organicvegetable material: wood, barks, twigs, wood pulp, fruit shells,especially coconut shells and groundnut shells.

The post-calcining treatments are aimed at eliminating the products(minerals, tars) which obstruct the pores.

The post-combustion treatments may be physical activation, whichinvolves a further combustion at high temperature, carried out in astream of air and steam, which are injected under pressure, which willproduce a narrow porosity on the surface of the carbon, substantiallyenhancing its surface area and its adsorbency.

Activation may also be carried out chemically, as for example usingnitric acid or phosphoric acid, leading to an active carbon havingrelatively large pores.

The diameter of the pores is also dependent on the pores which exist inthe raw material used. Very dense wood and coconut shells givemicropores (<2 nm), while medium white woods give mesopores (between 2and 50 nm) or macropores (>50 nm).

The surface area developed by the active carbon is immense: one gram ofactive carbon has a specific surface area of between 400 and 2500 m²/g,preferably between 500 and 1000 m²/g.

The iodine index, which defines the number of mg of iodine adsorbed perg of active carbon (ASTM D4607-94), is usually greater than or equal to1000 and is generally between 1000 and 1300.

The active carbons may be employed in the form of granules, especiallyextrudates, having a size ranging, for example, from 0.4 to 2 mm,preferably from 0.5 to 1.5 mm (ASTM D2862-97).

The invention does not rule out the use of carbon-containing adsorbentsformed, for example, by pyrolysis of polymeric resins.

Adsorbents suitable for the invention are obtained by pyrolysis ofmacrocrosslinked sulfonated styrene/-divinylbenzene ion-exchange resin.

Reference may be made in particular to U.S. Pat. No. 5,094,754. Suchadsorbents are sold by Rohm and Haas Company, under the registered trademark Ambersorb, as for example Ambersorb 563.

Another type of adsorbents suitable for the method of the invention arethe adsorbent polymers.

Adsorbent polymers generally take the form of beads having a porousstructure.

The base polymers are polystyrenes, more particularlystyrene-divinylbenzene copolymers, or are polyacrylics, especiallydivinylbenzene-acrylic ester copolymers, or are phenolic polymers, moreparticularly phenol/-formaldehyde copolymers.

The porosity is created by a high degree of crosslinking of the polymer.

Polymeric adsorbents generally have a specific surface area of from 100to 1000 m²/g, preferably between 400 and 800 m²/g.

They are generally mesoporous materials having a pore size of from 4 to60 nm: their internal porosity is between 0.4 and 1.2 cm³/g.

Preferred adsorbents are inert adsorbents containing no functionalgroups, and more particularly mesoporous copolymers of styrene anddivinylbenzene with a high degree of crosslinking and a high porosity.

Suitable more particularly for the implementation of the method of theinvention are the mesoporous adsorbent polymers of styrene anddivinylbenzene that are sold by Rohm and Haas under the trade nameAmberlite XAD, and more preferably the following polymers: Amberlite XADFPX66; Amberlite XAD 4; Amberlite XAD 16; Amberlite XAD 761; AmberliteXAD 1180N; Amberlite XAD 1600N; Amberlite XAD 18; Amberlite XAD7HP.

Other adsorbent polymers that may be mentioned include those sold underthe name Purolite Hypersol Macronet, Purolite Purosorb; and also thosesold by Lanxess, and more particularly Lewatit VP OC 1064 MD PH andLewatit VP OC 1163.

As other adsorbents, mention may also be made of zeolites.

A zeolite is a crystalline tectosilicate of natural or synthetic origin,in which the crystals result from the three-dimensional assembly oftetrahedral units of SiO₄ and TO₄, where T represents a trivalentelement such as aluminum, gallium, boron, and iron, preferably aluminum.

Aluminosilicate zeolites are the most common.

Within the crystalline network, zeolites have a system of cavities whichare connected to one another by channels having a well-defined diameter,referred to as the pores, with the channels forming a one-dimensional,two-dimensional or three-dimensional network.

In the method of the invention, it is preferred to employ zeoliteshaving a two-dimensional or three-dimensional network.

Although it is possible to employ a natural zeolite, preference is givento selecting a synthetic zeolite, which has constant physicochemicalcharacteristics.

It is advantageous to select a zeolite having an average pore diameterof greater than or equal to 6, and preferably of between 6 and 8 Å.

Examples of zeolites particularly suitable for implementing the methodof the invention include zeolites with a two-dimensional network, moreparticularly mordenite zeolites; zeolites having a three-dimensionalnetwork, more particularly β zeolites, and faujasite zeolites, moreparticularly Y zeolites; or mesoporous MCM zeolites.

As an indication, it will be specified that the average pore diameter ofthe mordenite, β and Y zeolites is of the order, respectively, of 6.5 Å,6.8 Å, and 7.2 Å.

In the various zeolites, the Si/Al ratio may vary widely.

Mordenites have a molar Si/Al ratio of 5 to 50.

β Zeolites have a molar Si/Al ratio of more than 8, preferably ofbetween 10 and 100, and more preferably of between 12 and 50.

The Y zeolites, especially the zeolites obtained after dealuminationtreatment (for example hydrotreatment, washing using hydrochloric acidor treatment by SiCl₄), have molar Si/Al ratios of more than 3,preferably of between 6 and 100.

Mesoporous MCM zeolites, more particularly MCM-41 and MCM-48, have molarSi/Al ratios of between 10 and 100, preferably of between 15 and 40.They have a BET specific surface area of between 700 and 1000 m²/g and apore diameter ranging generally between 15 and 40 Å.

Among all of these zeolites, it is preferred in the method of theinvention to employ β and Y zeolites.

The zeolites employed in the method of the invention are known productswhich are described in the literature [cf. Atlas of zeolite structuretypes by W. M. Meier and D. H. Olson published by the StructureCommission of the International Zeolite Association (1978)].

It is possible to employ commercially available zeolites or else tosynthesize them by the methods described in the literature, particularlyaccording to the references referred to in the aforementioned Atlas.

Also suitable for the method of the invention are molecular sieves.

Molecular sieves are synthetic zeolites which are characterized by acrystalline structure and a regular pore diameter.

They are metallic aluminosilicates which possess a three-dimensionalcrystalline structure composed of an assembly of SiO₄ and AlO₄tetrahedra, and containing cations such as Na⁺, K⁺ or Ca⁺⁺, to make thesystem electrically neutral.

The tetrahedra are assembled in such a way that they constitute atruncated octahedron. These octahedra are themselves arranged accordingto a simple cubic crystalline structure, forming a network with cavitieshaving an approximate diameter of 11.5 Å, these cavities beingaccessible via openings or pores.

The family of molecular sieves that is referred to commonly as MSincludes the 3A, 4A and 5A molecular sieves.

Molecular sieves of type A are characterized by a molar Si/Al ratio ofclose to 1.

The opening size of pores varies depending on the type of molecularsieve, since the pores may be blocked by cations and by cation exchange,and so it becomes possible to modify the size of the pores.

When these cations are derived from sodium, the pores have an openingdiameter of 4.1 Å, and the molecular sieve is then referred to as a 4Asieve.

Molecular sieve 4A is obtained by replacing a large part of the sodiumions with potassium ions, the diameter of the pores being approximately3 Å.

Molecular sieve 5A is produced by replacing the sodium ions with calciumions, the diameter of the pores in that case being of the order of 5 Å.

3A, 4A or 5A sieves are available commercially, in powder form andoptionally in the form of compositions with other substances, especiallya clay-based binder which may take the form of granules, beads orextrudates.

Among the various aforementioned sieves, sieve 5A is capable ofintervening preferentially in the method of the invention.

In accordance with the method of the invention, the reaction mixtureobtained at the end of the condensation reaction is passed onto theadsorbent support.

The stream is therefore at a temperature close to the condensationtemperature of between 20° C. and 60° C.

Generally speaking, the adsorbent support is placed in a stirred reactoror else in a column, with the mixture being introduced generally fromtop to bottom.

The amount of adsorbent support employed is determined on the basis ofthe adsorption efficiency of the adsorbent support.

As mentioned above, the adsorption efficiency varies generally between10% and 50%. Accordingly, in order to have complete adsorption, theamount of adsorbent employed is adapted: the lower the adsorptionefficiency, the higher the amount of adsorbent employed.

It will be specified that the amount of adsorbent represents at leastfrom 2 to 10 times the weight of the phenolic compound in salified formthat is to be adsorbed. It is preferred to employ an excess ofadsorbent, for example an excess of 10% to 20% of the weight ofadsorbent calculated.

At the column bottom an aqueous stream is recovered that comprises allof the mandelic compounds in salified form, while the phenolic compoundis adsorbed on the support.

The aqueous stream comprising the mandelic compounds in salified formmay be directly input into the oxidizing operation, thereby making itpossible to obtain the aromatic aldehyde corresponding to the mandeliccompounds in salified form.

In a subsequent step, the phenolic compound in salified form isrecovered by regeneration of the adsorbent.

Regeneration of the adsorbent by means of a base is selected withpreference.

Suitable bases include, in particular, sodium hydroxide or potassiumhydroxide.

For this purpose, a basic treatment is carried out, preferably using abasic aqueous solution, having a concentration of 1% to 10% by weight,and more preferably between 2% and 8% by weight. Sodium hydroxide istypically used.

The amount of base employed is at least equal to the amount of phenoliccompound in salified form that is to be regenerated.

A solution is thus obtained of the phenolic compound, which is salifiedand can therefore be recycled directly to the condensation step.

As mentioned above, the method of the invention makes it possible toseparate an aqueous stream comprising the various mandelic compounds insalified form.

Accordingly, the method of the invention allows access tohydroxyaromatic aldehydes corresponding to the formulae (IIa), (IIb),and (IIc) in which the glycol group of formula —CHOH—COOH is replaced bya formyl group CHO.

The oxidation reaction may be conducted according to the techniques thatare described in the literature. Thus it is possible to use thecatalysts that are conventionally used in oxidation reactions ofmandelic compounds in a basic medium.

The oxidation is generally conducted by oxygen or air under pressure, inthe presence of an appropriate catalyst such as, for example,derivatives of chromium, manganese, iron, cobalt, nickel, copper, zinc,bismuth, aluminum, silver, vanadium or osmium.

It should be noted that this list is not limitative.

It is possible, very particularly, to employ oxides, sulfates, halides,and acetates of said metallic elements.

It is also possible to use a catalyst comprising at least two metallicelements. Reference may be made more particularly to WO 2008/148760,which proposes the use of a catalyst system comprising at least twometallic elements, M₁ and M₂, which are selected from the groupconsisting of copper, nickel, cobalt, iron, and manganese.

Accordingly, the invention allows easy access to hydroxybenzaldehydes,and more particularly to vanillin and its analogs, for example,3-ethylvanillin and 3-isopropylvanillin, by oxidation, respectively, ofp-hydroxymandelic acid and of 4-hydroxy-3-methoxymandelic acid,3-ethoxy-4-hydroxymandelic acid, or 4-hydroxy-3-isopropoxymandelic acid.

Working examples of the invention are given below by way of illustrationand without any limitative character.

In the examples, the selectivity of the reaction is defined as thefollowing molar ratio:[guaiacol]adsorbed/([guaiacol]adsorbed+[mandelate] adsorbed).

EXAMPLE 1

In this example, the adsorbent used is 5A molecular sieve.

This solid is a calcium aluminosilicate. It possesses a pore diameter of0.5 nm.

A Schott tube is charged with 20 g of a stream comprising 0.95 g ofsodium guaiacolate (0.0065 mol) and 1.32 g of sodium p-mandelate (0.0055mol) of formula:

Then 2 g of 5A molecular sieve are added, and the reaction mixture isstirred at 40° C. throughout the duration of the adsorption.

The adsorption is monitored by high-performance liquid chromatographyanalysis of said solution.

After 15 minutes, 45% of guaiacolate is adsorbed.

After 1 hour 30 minutes, 55% by weight of guaiacolate is adsorbed(expressed relative to the total weight of guaiacolate present in thesolution to be treated), with no attachment of mandelates.

The ratio defining the selectivity is 1.

EXAMPLE 2

In this example, a Y zeolite is employed, namely the hydrophobic zeoliteWessalith DAY 55. The pore diameter of this zeolite is 0.72 nm.

Example 1 is repeated, with the only difference that the nature of theadsorbent is changed.

With this adsorbent, 44% of guaiacolate is adsorbed after 15 minutes,with no adsorption of mandelates.

The selectivity obtained is 1.

EXAMPLE 3

In this example, an adsorbent polymer is employed which is calledAmberlite XAD16, which is a copolymer of styrene and divinylbenzene andis a nonionic polymer. It is present in the form of beads, it possessesa large specific surface area of more than 700 m²/g, and its structureis very porous: the average pore size is 10 nm.

Example 1 is repeated, with the only difference that the nature of theadsorbent is changed.

With this adsorbent, the selectivity obtained is 1, and a 60% solutionof guaiacolate remains.

EXAMPLE 4

In this example, active carbons in the form of granules or extrudatesare employed as adsorbents.

The active carbons used have a high specific surface area (>1100 m²/g)and originate from the companies Norit, Ceca, and Eurocarb.

Example 1 is repeated, with the only difference that the nature of theadsorbent is changed.

The results obtained after 30 minutes are indicated in table (I).

TABLE I % of guaiacolate Specific adsorbed in Active surface relation tototal carbon used Origin area Selectivity weight of guaiacolate NoritROX 8 Coal 1225 1 70 Acticarbone Pine 1700 1 76 BGX260 Chemviron Coconut1300 1 81 CAL

EXAMPLE 5

In this example, the active carbon Eurocarb HT5 is employed, which isbased on coconut. It is present in the form of grains and possesses aspecific surface area of 1400 m²/g.

The carbon is employed in a 300 ml column with a diameter of 2.8 cm anda height of 24 cm.

The volume of carbon is 150 ml.

The column is maintained under atmospheric pressure at 35° C.

This carbon is percolated at a rate of 1.5 m/h with a stream comprising4.4% of sodium guaiacolate and 6.6% of mandelic compounds in sodium saltform (o-, p-, and di-mandelate), obtained from a condensation reactionbetween glyoxylic acid and guaiacol, in the presence of sodiumhydroxide, conducted according to the teaching of the prior art (WO99/65853), and corresponding to the formulae (IIa), (IIb), and (IIc) inwhich R represents a methoxy group, x is 1, and M is sodium.

In this example, 540 g of the stream as defined above are percolatedthrough the carbon.

At the outlet from the column, 340 g of a stream which is free fromsodium guaiacolate and contains all of the mandelic compounds in sodiumsalt form that were charged is recovered.

The ratio which defines the selectivity is 1.

The sodium guaiacolate adsorbed on the carbon is recovered by treatmentwith sodium hydroxide.

A 2% by weight aqueous solution of sodium hydroxide is percolatedthrough the carbon at a rate of 2 m/s.

A stream is recovered which contains sodium guaiacolate with a yield of60% by weight, the yield being defined as the weight ratio (in %)between the guaiacolate recovered and the guaiacolate introduced.

This guaiacolate may be directly recycled to the condensation step.

The stream at the column outlet that contains the mandelic compounds insalified form is then oxidized without further addition of aqueoussodium hydroxide solution.

The stream is charged to a 316L stainless-steel reactor equipped withmechanical stirring, baffles, and an air inlet.

This reaction mixture is admixed with a catalyst system comprisingCoCl₂.6H₂O and CuSO₄.5H₂O, which are employed, respectively, in anamount, expressed as molar percentage of mandelic compounds, of 0.125and 0.125.

The mixture is subsequently heated to 80° C. and air is introduced at arate of 1.6 L/h.

After 30 minutes of reaction, a selectivity of the reaction for vanillin(expressed by the ratio between the number of moles of vanillin formedand the number of moles of p-mandelate converted) of 98% is obtained.

EXAMPLE 6

Example 1 is reproduced, with the sole difference that the reactionmixture introduced results from the reaction of guaethol and glyoxylicacid in the presence of sodium hydroxide, and therefore comprises sodiumguaetholate and the mandelic compounds in sodium salt form (o-, p-, anddi-mandelate) which correspond to the formulae (IIa), (IIb), and (IIc)in which R represents an ethoxy group, x is 1, and M is sodium.

The adsorbent used in this example is the active carbon Eurocarb HT5.

After stirring for 1 hour and 30 minutes, a mixture is obtained whichcontains the entirety of the mandelic compounds, in the form of sodiumsalts, and 31% of sodium guaetholate.

With this adsorbent support, the ratio which defines the selectivity is1.

1. A method of separating phenolic compounds, the method comprisingseparating the phenolic compounds in salified form from an aqueousreaction mixture resulting from a reaction of a phenolic compound andglyoxylic acid in the presence of a base, leading to a reaction mixturecomprising at least an excess of phenolic starting compound in salifiedform and various mandelic compounds in salified form, resulting from thereaction, wherein the reaction mixture comprising the phenolic startingcompound in salified form is contacted with an adsorbent support,leading to selective adsorption of said phenolic compound on saidsupport, and to the recovery of an aqueous stream comprising themandelic compounds in salified form obtained from the reaction, and inthat the phenolic compound fixed on the adsorbent is desorbed by aregenerative treatment of said adsorbent.
 2. The method as defined byclaim 1, wherein the reaction mixture comprises a phenolic compound insalified form corresponding to the following formula:

in which formula: R is an alkyl or alkoxy group having from 1 to 4carbon atoms, or a halogen atom, x is a number from 0 to 3, and M is acation of a metallic element from group (IA) of the periodic table. 3.The method as defined by claim 1, wherein the phenolic compound insalified form of formula (I) is sodium guaiacolate or sodiumguaetholate.
 4. The method as defined by claim 1, wherein the reactionmixture comprises the phenolic compound in salified form at aconcentration of from 1% to 20% by weight.
 5. The method as defined byclaim 1, wherein the reaction mixture comprises reaction products whichare mandelic acids in salified form at a concentration of from 3% to 30%by weight.
 6. The method as defined by claim 1, wherein the reactionmixture is an aqueous mixture comprising sodium guaiacolate and mandeliccompounds: 4-hydroxy-3-methoxmandelic acid, 2-hydroxy-3-methoxymandelicacid, 4-hydroxy-5-methoxy-1,3-dimandelic acid.
 7. The method as definedby claim 1, wherein the adsorbent is selected from the group consistingof activated carbons, adsorbent polymers, zeolites, and molecularsieves.
 8. The method as defined by claim 7, wherein the adsorbent is anactivated carbon based on coal, peat, lignite, or petroleum distillationresidues, or on the basis of any carbon-rich organic vegetable matterfrom the group consisting of wood, barks, twigs, wood pulp, shells offruits.
 9. The method as defined by claim 7, wherein the adsorbent is acarbon-containing adsorbent resulting from the pyrolysis of a polymericresin.
 10. The method of claim 7, as defined by claim 7, wherein theadsorbent is an adsorbent polymer having a porous structure.
 11. Themethod as defined by claim 7, wherein the adsorbent is a polystyrenepolymer, a polyacrylic polymer, or a phenolic polymer.
 12. The method asdefined by claim 7, wherein the adsorbent polymer is selected from thegroup consisting of mesoporous adsorbent polymer of styrene anddivinylbenzene called Amberlite XAD, an adsorbent polymer calledPurolite Hypersol Macronet, Purolite Purosorb; and a polymer calledLewatit VP OC 1064 MD PH and Lewatit VP OC
 1163. 13. The method asdefined by claim 7, wherein the adsorbent is a zeolite having an averagepore diameter of greater than or equal to 6 Å.
 14. The method as definedby claim 7, wherein the adsorbent is a mordenite zeolite; a β zeolite, afaujasite zeolite, a Y zeolite; or a mesoporous MCM zeolite.
 15. Themethod as defined by claim 7, wherein the adsorbent is a molecularsieve.
 16. The method as defined by claim 1, wherein the amount ofadsorbent support employed represents at least from 2 to 10 times theweight of the phenolic compound in salified form that is to be adsorbed.17. The method as defined by claim 1, wherein the phenolic compound insalified form is recovered by regeneration of the adsorbent by a basictreatment, preferably by means of an aqueous sodium hydroxide solution.18. The method as defined by claim 2, wherein x is
 1. 19. The method asdefined by claim 2, wherein the cation of the metallic element fromgroup (IA) if the periodic table is selected from the group consistingof lithium, sodium, potassium, rubidium, cesium or an ammonium cation.20. The method as defined by claim 4, wherein the concentration ofsalified phenolic compound is at a concentration of from 5% to 10% byweight.
 21. The method as defined by claim 5, wherein the mandelic acidsare present in a concentration of from 5% to 20% by weight.
 22. Themethod as defined by claim 11, wherein the polystyrene polymer is astyrene-divinylbenzene copolymer.
 23. The method as defined by claim 11,wherein the polyacrylic polymer is a divinylbenzene-acrylic estercopolymer.
 24. The method as defined by claim 11, wherein the phenolicpolymer is a phenol formaldehyde copolymer.
 25. The method as defined byclaim 13, wherein the average pore diameter is from 6 Å to 8 Å.
 26. Themethod as defined by claim 15, wherein the molecular sieve is a 5 Åmolecular sieve.
 27. The method as defined by claim 17, wherein thephenolic compound is recovered by means of an aqueous sodium solution.